The present invention relates to a microactuator that can be fabricated according to the IC fabrication processes characterized by etching and lithography, for example, and can be used as a micropositioner for the multiple probe head of a scanning probe microscope and the pickup head for a signal recording and reproduction equipment.
As a first prior art example of a microactuator, there is an electrostatic micro wobble motor introduced in a paper authored by Mehregany et al. (xe2x80x9cOperation of microfabricated harmonic and ordinary side-drive motorsxe2x80x9d, proceedings of the third IEEE Workshop on Micro Electro Mechanical Systems, Napa Valley, Calif. USA, Feb. 11-14, 1990, pp. 1-8)
FIG. 50 is a schematic plan view to show how this prior art electrostatic micro wobble motor is structured and FIG. 51 is a cross-sectional view of the foregoing motor.
In FIG. 50 and FIG. 51, item 1 is a bearing, item 2 is a rotor of about 100 xcexcm in diameter and items 3a through 3h are eight electrodes arranged around the periphery of the rotor 2. (On a photograph of the motor in the above referenced paper, there are 12 static poles observed.) Although not shown in the drawings, these electrodes are connected by wires with a voltage supply source and can be applied with voltages arbitrarily selected.
As indicated in FIG. 50, the rotor 2 is shaped like a ring and between the inner circumference thereof and the bearing xe2x88x921 there exists a clearance C. Therefore, in contrast to an ordinary motor, the rotor 2 does not rotate around the bearing 1 with the bearing serving as the axis of rotation. As voltages are applied to electrodes 3a through 3h in succession, the rotor 2 revolves since it is attracted sequentially towards the excited electrodes 3a through 3h. 
At the same time, however, since the rotor 2 moves while it is in rolling contact with the bearing 1 at the contact point of 2a, the rotor 2 rotates by the difference between the outer circumference of the bearing 1 and the inner circumference of the rotor 2. A detailed description on this performance will be made later.
The rotor 2 is held by a flange 1a so that it does not slip off from the bearing 1. The electrodes 3a through 3h (only 3a and 3e are shown in FIG. 51) are almost of the same height as the rotor 2. On the bottom surface of the rotor 2, there are a plurality dot-like sounds 2b, not a ring-like mound, which slide on and contact electrically with a shield layer 4.
FIG. 52 through FIG. 56 are the cross-sectional illustrations to show the fabrication processes (a) through (e), respectively, for the electrostatic micro wobble motor, which will be described hereunder. The fabrication processes employ the ordinary IC fabrication methods such as etching, lithography or the like.
(a) As illustrated in FIG. 52, an insulating layer 6 is first formed on a silicon substrate 5 by depositing in succession an oxide layer of 1 xcexcm in thickness grown thermally and a silicon nitride layer of 1 xcexcm in thickness formed by means of a low pressure chemical vapor deposition method (LPCVD).
Then, a polysilicon thin film of 3500 xc3x85 thick with phosphorus diffused sufficiently therein is formed by LPCVD and patterning is applied thereto to complete an electric shield layer 4.
Further, a low temperature oxide layer (LTO) 7 of 2.2 xcexcm thick is deposited to make a first sacrificial layer and then patterning is applied by 2 steps, the one for forming a base 7a of the electrodes 3a through 3h and the other for forming a hollow 7b in preparation of creating a mound 2b on the bottom of the rotor 2.
(b) As illustrated in FIG. 53, a polysilicon layer of 2.5 xcexcm thick diffused with phosphorus sufficiently is deposited by LPCVD and then the rotor 2 and the electrodes 3a through 3h (only 3a and 3e are shown here) as indicated in FIG. 50 and FIG. 51 were formed by means of a reactive ion etching method (RIE). As shown in FIG. 53, the electrodes 3a through 3h are fixed on the silicon substrate 6 and a plurality of the mound 2b are formed on the bottom of the rotor 2. On account of a thermal oxidation layer after patterning used as the mask for the reactive ion etching of the foregoing polysilicon layer, the thickness of the rotor 2 as well as the electrodes 3a through 3h is approximately 2.2 xcexcm at this stage.
(c) As illustrated in FIG. 54, an LTO layer 8 to make a second sacrificial layer of about 0.3 xcexcm thick is deposited for retaining the clearance C between the bearing 1 and the rotor 2. At the same time, an anchor 8a for the bearing 1 is formed by patterning.
Although the diameter of the bearing 1 is about 36 xcexcm, the smallest possible diameter is 26 xcexcm due to the restrictions imposed by the process employed here.
(d) As illustrated in FIG. 55, a polysilicon layer of 1 xcexcm thick with phosphorus diffused sufficiently is deposited by LPCVD and the bearing 1 provided with the flange 1a is formed.
(e) As illustrated in FIG. 56, the LTO layers 7 and 8 serving as the first and second sacrificial layers respectively are dissolved by buffered hydrogen fluoride (HF) and the rotor is released completely to realize the structure as shown in FIG. 51.
The operational principle of the prior art electrostatic micro wobble motor having a structure as described above will be explained in the following with the help of FIG. 50. As stated before, the rotor 2 does not rotate around the bearing 1 with the bearing serving as the axis of rotation. Instead, the rotor revolves as it is attracted by the excited electrodes 3a through 3h sequentially and at the same time it rotates by the difference between the outer circumference of the bearing 1 and the inner circumference of the rotor 2 while it is in rolling contact with the bearing 1 at the contact point 2a. 
In other words, suppose the electrodes are excited in the direction X as indicated in FIG. 50 in an order of the electrodes 3a, 3b, 3c and so forth, then the rotor 2 is first attracted by the excited electrode 3a. Next, it will be attracted by the electrode 3b and then by the electrode 3c and so forth, resulting in revolving of the rotor 2 also in the direction X.
Since the clearance C between the rotor 2 and the bearing 1 is set up to be smaller than the gap between the rotor 2 and the electrodes 3a through 3h, there will be the contact point 2a where the rotor 2 will come into contact with the bearing 1. Besides, the correct gap between the rotor 2 and the electrodes 3a through 3h corresponds to G+E as indicated in FIG. 50, where the E means an effective gap length that produces the motor""s torque, and the following relationship is established inherently from the structure of the motor:
E=Gxe2x88x92C greater than 0
(This is obvious when the state of the electrode 3e being excited has been observed.) As the gap between the rotor 2 and the electrodes 3a through 3h, G will be dealt with for convenience because of the possibilities in reducing the effective gap length E to the minimum.
Now, as the rotor 2 revolves in the direction X, the contact point 2a also is to move likewise in the direction X. Since the bearing 1 is fixed in position, slipping at the contact point 2a by the amount of the difference between the outer circumference of the bearing 1 and the inner circumference of the rotor 2 is taking place unless the rotor 2 revolves. However, attracting force is applied to the rotor 2 in the direction of pressing the bearing 1 and practically any slipping hardly occurs at the contact point 2a. 
Therefore, as the rotor 2 revolves in the same direction as the shifting direction (the direction of X) of the voltages applied to the electrodes 3a through 3h, the rotor 2 is consequently to rotate in the same direction (the X direction of FIG. 50) by the amount corresponding to the difference between the outer circumference of the bearing 1 and the inner circumference of the rotor 2. It is needless to say that the contact point 2a is to move in the direction X while it is in the state of rolling contact.
The feature of this electrostatic micro wobble motor is in that the revolution frequency of the revolving rotor 2 is determined by the outer diameter B of the bearing 1 and the inner diameter R of the rotor 2, and the revolution frequency S of the revolving rotor 2 is to become extremely small against the shifting frequency F of the input voltages (the physically shifting frequency of the voltages applied to the electrodes 3a through 3h for the present prior art example) when compared with the case of an ordinary motor.
In case wherein the revolution frequency of the revolving motor 2 is equal to the shifting frequency F of the input voltages, the revolution frequency S of the revolving rotor 2 is expressed by the following equation:
S=Fxc3x97(Rxe2x88x92B)/R=Fxc3x97C/R
Since the revolution speed is reduced to S/F, e.g. C/R, the torque will be increased, instead, to R/C times in contrast to the case of an ordinary motor. In addition, by reducing the clearance C, the slippage at the contact point 2a can be eliminated, resulting in advantageously suppressing the wobbling of the rotor 2 caused by its revolution.
An a result, a motor of low speed and high torque has been realized without using any speed reduction means in particular and greatly expected to be used as the motive force or the like for micromachines.
On the other hand, the scanning probe microscopes, typically represented by STM (Scanning Tunneling Microscope), have been prevailing widely and rapidly in recent years as a means to observe minute objects on the surface of specimens for the high resolution and the feature that the microscopes can be used under any measurement environments in principle. Especially, the progress in development of the STM has been remarkable and many research studies have been made on the methods to produce probing needles and cantilevers by fine fabrication processing of silicon. These efforts have been aiming at down-sizing of the equipment by micro-miniaturizing the mechanical parts involved and also improvement of the vibration resistant characteristics by increasing the resonant frequencies of the mechanical parts.
These probing needles and cantilevers are generally referred to as probes and usually driven finely by piezo elements separately prepared. Therefore, the dimensions of the whole mechanical parts are mostly accounted for by the dimensions of piezo elements even when the probes are reduced in size.
Studies have been recently started to create a thin film piezo element on a cantilever for micro-deforming the cantilever itself and some study results have already been made public.
As a second prior art example of a microactuator, there is a thin film probe which has been used as a probe head for scanning probe microscopes and was introduced by a paper authored by Akamine et al. (xe2x80x9cA planar process for Micro-fabrication of a Scanning Tunneling Microscopexe2x80x9d, Sensors and Actuators, A21-23, pp. 964-970, 1990)
FIG. 57 shows how the aforementioned prior art thin film probe is structured.
In FIG. 57, item 101 is a silicon substrate and item 102 is a cantilever, on the end point of which a probing needle 103 is being attached. The cantilever 102 measures 8 in thickness by 200 in width by 1000 xcexcm in length.
The cantilever 102 is fundamentally of a bimorph structure consisting of thin film piezo elements 104 and 105. On the upper surface of the thin film piezo element 104 are formed electrodes 106a through 106c and on the bottom surface of the thin film piezo element 105 are formed electrodes 107a and 107b, which are almost identical in configuration with the electrodes 106a and 106b, respectively. Besides, an electrode 108 is formed between the thin film piezo elements 104 and 105 and also the probing needle 103 is fixed on the electrode 106c. 
As illustrated in FIG. 57, electrical wirings are provided to connect each of the electrodes 106a through 106c, 107a, 107b and 108 respectively with pads, through which arbitrary voltages can be applied to the electrodes.
FIG. 58 through FIG. 62 are cross-sectional illustrations of the fabrication processes (a) through (b) for the above thin film probe, wherein the generally known semiconductor processes such as etching, lithography or the like are utilized. With the help of the foregoing illustrations, the fabrication processes will be explained briefly hereunder.
(a) As shown in FIG. 58, a membrane 109 of 50 to 70 xcexcm thick is formed through an application of anisotropic etching to the bottom surface of the silicon substrate 101.
(b) As shown in FIG. 59, electrodes 107a and 107b (not shown in FIG. 59) are formed by deposition of a first Al thin layer to a thickness of 0.5 xcexcm by means of electron beam evaporation and then by patterning thereof.
(c) As shown in FIG. 60, a thin film piezo element 105 is formed by successive deposition of a first nitride layer of 0.2 um thick by means of plasma-enhanced chemical vapor deposition (PECVD), a first zinc oxide layer of 3 xcexcm thick by reactive sputtering and then a second nitride layer of 0.2 xcexcm thick on the electrodes 107a and 107b. 
Patterning is performed with the nitride layer by means of plasma etching and with the zinc oxide layer by wet etching, respectively.
(d) As shown in FIG. 61, a thin film piezo element 104 is formed by successive deposition of a second Al thin layer serving as electrode 108 according to the same steps as (b), and a third nitride layer, a second zinc oxide layer and a fourth nitride layer by means of PECVD and reactive sputtering according to the same steps as (c) on the foregoing thin film piezo element 105. Further, electrodes 106a through 106c (only 106a is shown in FIG. 61) are formed by deposition of a third Al layer according to the same steps as (b) on the thin film piezo element 105.
(e) As shown in FIG. 62, the membrane 109 is lastly removed from the bottom surface of the substrate by plasma etching to complete the structure as indicated in FIG. 57.
Next, how this thin film probe operates will be explained briefly with the help of FIG. 63 through FIG. 66.
As is generally known, a piezo element has a property of expansion or contraction depending on the direction of the electric field applied thereto. Therefore, by applying an appropriate voltage of either positive or negative polarity to the electrodes 106a, 106b, 107a and 107b while the electrode 108 is kept grounded, the cantilever 102 can be freely deformed through a control of the electric field that is applied to the thin film piezo elements 104 and 105.
As illustrated in FIG. 63 for example, when voltages of the same polarity are applied to the electrodes 106a, 106b, 107a and 107b and also electric fields of the same direction are applied to the thin film piezo elements 104 and 105, the cantilever 102 as a whole will show expansion or contraction in the longitudinal direction. (X direction in FIG. 63).
Also, as illustrated in FIG. 64 through FIG. 66 by hatched lines (indicating that closely spaced diagonal lines rising towards right mean the applied voltage to be positive, for example, and loosely spaced lines rising towards left mean the applied voltage to be negative), when the electric fields applied to the thin film piezo elements 104 and 105 are controlled by applying a positive or a negative voltage to each respective electrode, it will be possible to provide the end of the cantilever 102 with such motions of high freedom as moving in the horizontal direction (the Y direction in FIG. 64) or in the vertical direction (the Z direction in FIG. 65) or twisting as indicated by an arrow M in FIG. 66.
Accordingly, the probing needle 103 attached to the end of the cantilever can be precisely moved for the purpose of scanning a specimen since the piezo elements have extremely high resolution.
Thus, a thin film probe for the scanning probe microscopy can be made by depositing piezo elements on a silicon substrate through semiconductor fabrication processes and it is expected that the employment of this probe will greatly contribute to the production of an extremely small and high performance scanning probe microscope.
Also, magnetic heads for VTR and magnetic disc equipment and optical heads for optical disc equipment are generally known as the conventional pickup heads for recording and reproducing equipment. Efforts have been always made to make the recording and reproducing equipment smaller in size and larger in recording capacity. For that purpose, development and progress of the precision mechanism technology as typically applied to high density recording and pickup heads is absolutely necessary.
There is a probe recording method among many new approaches proposed at present for the high density recording. This method is to use a probing needle as used with a STM or the like as a head for mechanical scanning.
The probing needle type charge storage recording, for example, as described in a paper authored by Barrett et al. (xe2x80x9cCharge storage in a nitride-oxide-silicon medium by scanning capacitance microscopyxe2x80x9d, J. Appl. Phys. Vol. 70, No. 5, pp. 2725-2733, Sep. 1, 1991) proves the possibility of high density and non-destructive recording and reproducing.
The principle of operation thereof will be explained here briefly. When an electro-conductive probing needle mounted on a cantilever of an AFM (Atomic Force Microscope) is kept in contact with a dielectrics layer (silicon nitride), which is placed on an electro-conductive body, and applied with a bias voltage, electric charges will be trapped by the dielectrics layer and information will be stored therein. Reproduction of the information is performed by detecting capacitance existent between the probing needle and the substrate by means of a sensor. It is possible to erase the information by applying a reversed bias voltage and also perform recording repeatedly.
The recording medium is prepared by depositing an oxide layer and a nitride layer on a polysilicon substrate which was added with boron. By having a probing tip of tungsten placed in contact with the above recording medium and a voltage of xe2x88x9225 V applied for 20 xcexcsec., information of 75 nm bits have been recorded. The recording density has reached as many as 180 bits/xcexcm2, more than 200 times the conventional recording.
The first prior art structure as exemplified in the foregoing tends to have torque loss due to rolling friction since the dot-like mounds on the rotor bottom are in contact with the shield layer and also the flange is in contact with the rotor while the rotor is revolving. Besides, various parts of the motor suffer from mechanical wears due to friction over a long period, resulting in a considerable reduction in the life of the motor.
Further, the rotor and the shield layer tend to fail in having the respective potentials kept at the same level in a stable manner since the electrical contact between them is performed through a sliding action between the dot-like mounds under the rotor bottom and the shield layer surface, resulting in deteriorated reliability.
Furthermore, since the rotational characteristics of the motor are governed by the dimensional accuracy of the outer diameter of the bearing, the outer and inner diameters of the rotor, and the inner diameter of the electrode lay-out, it is difficult to gain stable rotational accuracy.
In addition, the difficulty in taking out the torque of the rotor for possible utilization in a micropositioner on account of the rotor moving only in the inner space inside the boundary formed by the electrode lay-out has presented a problem.
Also, It is required according to the second prior art structure as exemplified in the foregoing to move either the entire thin film probe including the silicon substrate 101 or the specimen in order to perform a measurement of a different place of the specimen.
Besides, there has been a problem of not being able to observe a particular surface of a specimen multi-purposely by not only an STM but also, for example, an AFM or a MFM (Magnetic Force Microscope).
The conventional precision mechanism technology has been mostly involved with component parts in the areas of how to put them together, realizing higher accuracy of them, making them smaller in dimension, laying them out effectively as a whole and so forth. The problem has been in that even when an innovative progress was made in the recording principle or in the recording medium such innovation was not utilized to the fullest extent in achieving the ultimate miniaturization and performance of the recording and reproducing equipment.
The object of the present invention is to provide a microactuator that can be produced by semiconductor processes, which excel in micro-miniaturization and mass-producibility, is capable of high accuracy positioning and also in possession of long life and high reliability.
The structure disclosed by the present invention comprises:
a plurality of electrodes arranged along a circumference on a substrate;
a ring-like displacement plate located inside said electrodes;
beams, each of which is fixed at one end to an anchor solidly formed on said substrate and at the other end to a specified place of the inner circumference of said displacement plate in support of said displacement plate elastically; and
a voltage application means whereby voltages are selectively applied to said respective electrodes to have said displacement plate attracted electrostatically towards said electrodes, which have been applied with said voltages, and moved.
The foregoing structure as disclosed by the present invention has made it possible to bring about the following effects:
Since the displacement plate to be driven electrostatically is securely in electrical contact through the beams, reliability in the driving characteristics of the displacement plate has been greatly enhanced.
The adverse effect due to friction at the time when the displacement plate comes into contact with the electrodes has been made extremely small, resulting in longer life.
It has been made possible to control the position and angle of the displacement plate very accurately with resultant realization of an excellent positioning mechanism.
High torque has been obtainable.
It has become possible to design a microactuator for the most suitable configuration with abundant freedom.
With the use of semiconductor processes, it has become possible to achieve micro-miniaturization and secure mass-producibility.
Besides, by employing the microactuator of the present invention in a multi-head probe for a scanning probe microscope, a plurality of movable thin film probes can be simultaneously manipulated to realize equipment whereby a surface of a specimen is observed in a diversified manner without moving the specimen.
Furthermore, by utilizing the microactuator of the present invention in a pickup head of recording and reproducing equipment, it has become possible to provide a microactuator produced by an entirely new fabrication method based on a concept that has not been existent before.